1. Field of the Invention
The present invention relates to a circuit which corrects a mismatch (phase error and amplitude error) between an I-phase (in-phase) signal and a Q-phase (quadrature) signal outputted from a quadrature receiver in a digital communication system.
2. Description of the Related Art
In a standard receiver in a wireless or wireline digital communication system, a high-frequency signal including modulated information is demodulated to extract a desired information signal. The extracted information signal, generally being a complex value, is called a baseband signal, and has an I-phase (in-phase) component and a Q-phase (quadrature) component. The I-phase and Q-phase signals are modulated on a carrier wave to be 90 degrees apart and to have orthogonality on a transmitter side. Furthermore, the I-phase and Q-phase signals generally have the same electric power characteristics and have orthonormality. An orthogonal receiver is generally used to extract a baseband signal from a received signal.
An orthogonal receiver 100, as shown in FIG. 9, includes a receiving interface 101 to receive a transmission signal transmitted from a transmitter side through a wireless or wireline communication channel, a circuit 102 to separate a reception signal SR outputted from the receiving interface 101 into two I-phase reception signal SRI and Q-phase reception signal SRQ which are orthogonal to each other, and low-pass filters 107 and 108 which remove high-frequency components from the I-phase reception signal SRI and the Q-phase reception signal SRQ. The circuit 102 includes two mixers 103 and 104, a local oscillator 105, and a phase shifter 106 which shifts a phase of an oscillation signal SOL in the local oscillator 105 by 90 degrees. The mixer 103 multiplies the reception signal SR by an oscillation signal SL in the local oscillator 105 to generate the I-phase reception signal SRI, and the mixer 104 multiplies the reception signal SR by an oscillation signal SLQ in the phase shifter 106 to generate the Q-phase reception signal SRQ. An I-phase baseband signal SI and a Q-phase baseband signal SQ outputted from the low-pass filters 107 and 108 are digitized by A/D converters 109 and 110, respectively, to generate I-phase and Q-phase digital baseband signals SDI and SDQ.
In an ideal communication link, theoretically, the digital baseband signals SDI and SDQ have complete orthonormality, and can be directly processed by a demodulation circuit suitable for a predetermined communication link.
However, in an actual circuit, various defects in a transmitter, a receiver, or both of them which are present on a transmission path lead to a loss of the orthonormality between the I-phase baseband signal SI and the Q-phase baseband signal SQ. In the loss of the orthonormality (referred to as “IQ mismatch”), the I-phase baseband signal SI and the Q-phase baseband signal SQ is considered to interfere with each other. Due to the IQ mismatch, severe distortion occurs between an original baseband signal on a transmitter side and a baseband signal outputted from a receiver. This distortion which damages the communication link in quality and usability, the loss of the orthogonality (phase mismatch or phase error), and an energy difference (gain mismatch or amplitude error) must be corrected to ensure that a distortion level at an output terminal of the receiver is kept within tolerance and is not detrimental to the quality and usability of the communication link.
FIGS. 10A and 10B show an example of quality deterioration of a reception signal by an IQ mismatch. FIGS. 10A and 10B show quality deterioration caused by an IQ mismatch in a PAL video signal. FIG. 10A shows an electric power spectrum density (PSD) of an ideal signal obtained before the IQ mismatch occurs, and FIG. 10B shows an electric power spectrum density of a signal obtained after the IQ mismatch occurs. In FIG. 10A, peaks of a video carrier signal 201 and a sound carrier signal 202 can be confirmed. In contrast to this, in FIG. 10B, due to the presence of interference spectrum elements 203 and 204 except for the two peaks mentioned above, deterioration in signal quality can be confirmed.
In actual system design, the IQ mismatch is caused by a factor unique to an electronic part or an electronic device which cannot be easily controlled. In order to preferably maintain communication link capability, a circuit (IQ mismatch correction circuit) to correct an IQ mismatch is required to have a sufficient capability to cope with an IQ failure which may be caused by a predetermined design under a desired operation condition.
In general, an IQ mismatch correction circuit used in an actual system can correct predetermined mismatches (phase mismatch and gain mismatch) over a whole communication signal band. In other words, a conventional IQ mismatch correction circuit cannot correct a frequency-dependent phase IQ mismatch. In a receiver related to a multi-carrier communication system, a system having an additional adjustment circuit, or the like, there is disclosed a system which can moderate a frequency-dependent IQ mismatch to some extent. However, since the system requires several assumptions on quality of a transmission signal or requires a large-scale additional circuit which is a factor that increases the complexity of the system and the cost of the system, the system is not sufficiently matched with a general receiver.
The problem of the IQ mismatch correction circuit is known to a person skilled in the art such as designers or the like of receivers. In various applications, several methods and circuits to remove or attenuate an IQ mismatch are devised. However, most of these methods and circuits are based on the assumption that the IQ mismatch is not frequency-dependent. That is, the IQ mismatch is constant over a whole predetermined communication signal band. As a main drawback of the assumption, the corresponding circuit and system cannot correct a frequency-dependent IQ mismatch. Many communication links characterized by devices and parts having IQ mismatch characteristics in which a variation of the IQ mismatch is so large in a communication path band that the IQ mismatch must depend on a frequency can be experimentally observed. In other words, an IQ mismatch correction circuit which can preferably correct a frequency-dependent IQ mismatch is desired. In this case, it must be understood that a frequency-dependent IQ mismatch is general and that an IQ mismatch which is not frequency-dependent is rather special and limited. A correction circuit which can correct a frequency-dependent IQ mismatch can also correct an IQ mismatch which does not depend on a frequency in the same manner.
Conventional IQ mismatch correction techniques and methods are disclosed in U.S. Pat. Nos. 5,157,697, 5,705,949, 6,330,290, 6,898,252, 7,158,586, 7,274,750, 7,298,793, and Koji Maeda et al., “Wideband Image-Rejection Circuit for Low-IF Receivers”, ISSCC 2006, 26. 1. For example, U.S. Pat. No. 5,705,949 discloses digital circuit design which can correct an IQ mismatch which is not frequency-dependent without a connection to another part of a receiver. In order to correct a frequency-dependent IQ mismatch, U.S. Pat. No. 6,330,290, U.S. Pat. No. 6,898,252, or Koji Maeda et al., “Wideband Image-Rejection Circuit for Low-IF Receivers”, ISSCC 2006, 26. 1 discloses a technique which generates an adjusting signal in an analog circuit of a receiver or on a transmitter side, i.e., a technique which requires an additional circuit besides digital signal processing on a receiver side. The conventional technique disadvantageously requires the additional circuit. In addition, the conventional technique is disadvantageous in that the technique cannot applied to a system in which an analog circuit of a receiver and a digital circuit which corrects an IQ mismatch are separated from each other because, for example, the analog circuit and the digital circuit are supplied from different manufacturers. Furthermore, the conventional technique is disadvantageous in that, when a mismatch condition changes, the technique cannot cope with the conditional change without stopping reception for readjustment. More specifically, even though an adjustment signal is put in an analog circuit, the conventional technique cannot follow the change of the mismatch condition while receiving a predetermined communication signal.